Imaging in X-ray diagnosis requires image detectors which make it possible to record and visualize X-ray radiation. Image detectors that are in widespread use are films sensitive to X-rays, which are exposed by the impinging X-ray radiation and have to be developed in order to generate an image after the exposure.
In the course of general image digitization, but in particular also with regard to computer tomography, there is increasingly a demand for image detectors which allow digital images to be generated without a circuitous route via a conventional film recording. In order to ensure a corresponding image resolution, they have to be embodied with a large area and with many lines or many pixels.
In computer tomography, use is made for this purpose of photodiodes which detect the radiation from scintillators which are structured in a complicated manner and convert the X-ray radiation into a radiation of changed wavelength. In other imaging methods, use is also made of a-Si diodes, so-called FD diodes, which detect the radiation of so-called luminescent material layers based on cesium iodide and titanium. Moreover, semiconductor detectors are also known in which the X-ray radiation itself can be detected directly, that is to say without any prior change to the wavelength by a luminescent material layer. FD diode detectors, scintillator detectors and semiconductor detectors have to be structured in a complicated manner. As a result, the known image detectors are complicated in their construction and expensive to produce. Furthermore, they do not permit energy-resolved detection of the X-ray radiation.
An image detector which is less complicated to produce and is thus less expensive is based on the use of organic photodiodes. A diode of this type is proposed in WO 99/09603 in order to provide an inexpensive alternative, which can be produced with a large area, to conventional inorganic-based image detectors. In that case it is presupposed that the radiation sensitivity of organic photodiodes is sufficient to be able to produce image detectors for both color and black/white images. What is proposed for the detection of such images is an organic-based photodiode whose light sensitivity can be controlled by the application of an electrical voltage. While the sensitivity of the diode proposed is sufficient for visible light, this is not the case for X-ray radiation. Moreover, the sensitivity cannot be increased sufficiently by the use of luminescent material layers.